Passive layer

The so-called passive layer is a thin oxide film that forms on the surface on non-corrosive steel when it comes into contact with oxygen. The chromium atoms of the steel subsequently form a thin and inert (hence, the name "passive layer") oxide film that prevents the oxidation from proceeding, thereby preventing the steel from corroding. The characteristic and durability of the passive layer depends primarily on the alloy composition of the steel.

Stainless steels react to oxide and form an oxide film to the same degree as normal steel. In normal steel, the oxygen reacts with the iron atoms existing in the steel; this reaction produces a porous surface that allows the reaction to proceed. This process can result in the complete "rusting out" of the work piece.

In non-corrosive steel, the oxygen reacts with chromium atoms, which are contained in the steel at a relatively high concentration, to form the passive layer on the surface.
There are two reasons for the formation of rust on "non-corrosive" stainless steels:

the passive layer could not form, or

the passive layer was damaged

The inability of the passive layer to form can only be prevented by a high degree of cleanliness. All processed surfaces must be thoroughly cleaned of all residue. This applies, in particular, to any residue left behind by abrasives. For this reason, all abrasives suitable for processing stainless steel are free (mass portion < 0.01 per cent) of chloride, iron and sulphur.